Every year or two we run >32-bit vs. 64-bit Linux benchmarks. While x86_64 Intel/AMD hardware has been extremely common for quite some time, we continue to be amazed at the number of people still running an i686 Linux distribution on x86_64 hardware.
At least in the past year or two we've seen most Linux distributions finally endorse their x86_64 ISOs as their preferred installation media over i686. Some distributions have also worked to demote their i686 spins and some have stopped producing 32-bit x86 install media entirely. Fortunately, there's also been little reason these days for x86_64 hardware owners to consider using a 32-bit Linux distribution with Adobe Flash, Java, Linux gaming, and other past challenges having been settled in a 64-bit Linux world. About the only logical reason left for some x86_64 owners to still use 32-bit Linux is if you have less than 2GB of system memory, but even then for those older systems currently at 2GB of less of RAM, DDR2/DDR3 is incredibly cheap these days.